drm/i915/display/psr: Fix cppcheck warnings

Fix redundant condition, caught in cppcheck by kernel test robot.

Reported-by: kernel test robot <lkp@intel.com>
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Fixes: b64d6c51 ("drm/i915/display: Support PSR Multiple Instances")
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Harish Chegondi <harish.chegondi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210409231738.238682-1-jose.souza@intel.com